Computer supported or machine aided indexing (MAI) can be categorized in multiple ways. The system used by the National Aeronautics and Space Administration's (NASA's) Center for AeroSpace Information (CASI) is described as semantic and computational. It's based on the co-occurrence of domain-specific terminology in parts of a sentence, and the probability that an indexer will assign a particular index term when a given word or phrase is encountered in text. The NASA CASI system is run on demand by the indexer and responds in 3 to 9 seconds with a list of suggested, authorized terms. The system was originally based on a syntactic system used in the late 1970's by the Defense Technical Information Center (DTIC). The NASA mainframe-supported system consists of three components: two programs and a knowledge base (KB). The evolution of the system is described and flow charts illustrate the MAI procedures. Tests used to evaluate NASA's MAI system were limited to those that would not slow production. A very early test indicated that MAI saved about 3 minutes and provided several additional terms for each document indexed. It also was determined that time and other resources spent in careful construction of the KB pay off with high-quality output and indexer acceptance of MAI results.